Thermosetting type resin compositions such as epoxy resins have been widely employed as adhesive materials, molding materials, and the like. An imidazole-based latent curing agent has been employed as one of curing agents for such compositions. Such an imidazole-based latent curing agent does not exhibit curing ability under normal storage conditions and thus has been widely employed in order for a thermosetting epoxy resin composition to be used as a one-component type curable composition having good handleability and good storage stability. As a representative example of such an imidazole latent curing agent, a microcapsule-type imidazole latent curing agent is known in which the surface of imidazole compound particles having an ability to cure epoxy resin is coated with a cured epoxy resin material.
However, in order to initiate curing reaction, such a microcapsule-type imidazole latent curing agent must be pressurized and heated to 180° C. or higher since the coating thereof is relatively stable mechanically and also thermally. Therefore, a problem exists in that such a latent curing agent is unable to cope with recent low temperature curing type epoxy resin compositions.
Hence, as a latent curing agent exhibiting low-temperature fast-curing activity without using a toxic promoter such as antimony, a microcapsule-type aluminum chelate-based latent curing agent has been proposed (Patent Document 1). In this latent curing agent, fine particles of polyvinyl alcohol (child particles) are made to melt-adhere to the surface of particles of an aluminum chelate agent (parent particles) by means of a hybridization method to thereby form a polyvinyl alcohol coating layer on the surface of the parent particles. Here, the aluminum chelate agent co-operates with silanol (a silane coupling agent or the like) serving as a co-catalyst to generate a protonic acid and thus is capable of polymerizing cyclic ethers (epoxy compounds and oxetane compounds) through cationic ring-opening polymerization, and the above polyvinyl alcohol has a hydroxyl group which reacts with the aluminum chelate agent. In addition, another microcapsule-type aluminum chelate-based latent curing agent has been proposed (Patent Document 2). In this latent curing agent, fluororesin-based fine particles (child particles) not having a functional group capable of reacting with an aluminum chelate agent are made to electrostatically adhere to parent particles, and subsequently these fluororesin-based fine particles are fused by means of a hybridization method to thereby form a coating layer on the parent particles.
The detail of a curing step by means of the aluminum chelate-based latent curing agent is described in paragraphs [0007] to [0010] in the abovementioned Patent Document 1.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-368047    [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-363255